1. Field of the Invention
The present invention relates to an image processing method and a system for radiological diagnostics, which employs an X-ray digital diagnosis image in order to enhance the precision of radiological diagnosis.
2. Description of the Related Art
As a conventional system for radiological diagnostics, a digital subtraction angiography (DSA) system has been well known. In the DSA system, a digital subtraction image can be obtained by using a mask image obtained without the use of a contrast medium and a contrast image obtained with the use of a contrast medium. With use of the DSA system, angiography or ventriculogram is performed, and a densitometry method is used, in order to image the diameter of a blood vessel or measure the volume of a ventricle. A radiographic image (hereinafter, referred to as "X-ray image" since the X-ray image is a typical radiographic image) obtained by this system is not precise, since it is mixed with X-ray measurement error beam components due to X-ray beams scattering on the subject and beam hardening effect.
Regarding these error components, a method of correcting error components due to X-ray beam scattering has been put into practice, the error components have been corrected satisfactorily. The beam hardening effect depends on the energy absorption coefficient of a material (subject), through which the beam passes, and a thickness of the material. Suppose that water is used as material. For example, when the water thickness is 10 cm, the mean absorption coefficient thereof is 0.25. On the other hand, when the water thickness is 15 cm, the mean absorption coefficient is 0.24. Therefore when the water thickness is 10 cm, the X-ray permeability is e.sup.-0.25.times.10, and when the water thickness is 15 cm, the X-ray permeability is e.sup.-0.24.times.10.
The mean absorption coefficient is changed by the beam hardening effect, in accordance with the change in water thickness. Thus, the relationship between the material thickness (water thickness) and the X-ray permeability is not linear. More specifically, the density of an obtained image of a part of the subject which has a larger thickness, for example, the chest, is not equal to the density of an image of another part. The beam-hardening effect adversely affects the measurement error of the density of X-ray images.